Nominal power
Encyclopedia
Nominal power is a measurement
of a mediumwave
radio station
's output used in the United States
. AM broadcast
ers are licensed by the Federal Communications Commission
to operate at a specific nominal
power
, which may be (and usually is) different from the transmitter power output
.
In both cases, nominal power excludes losses in transmission lines between the tower or phasor
and the transmitter; however, it includes losses in a resistor network used to decrease the efficiency of the antenna system.
Nominal power is ultimately a historical artifact of the regulatory regime employed by the FCC prior to the 1980s. In the old system, rather than allowing licensees to choose any power level which would meet the efficiency and interference
standards for their class, stations were restricted to a small set of power levels: 50, 100, 250, 500, 1000, 2500, 5000, 10000, 25000, and 50000 watt
s. A station whose maximum coverage would otherwise be available at 4500 watts (given a specific directional pattern and antenna system efficiency) had a choice of either living with 2500 watts, or reducing the antenna efficiency to a level which would allow for 5 kW. Newly-constructed stations could fairly easily design an antenna system to meet the requirements, but stations on or moving to a shared tower with higher efficiency had a problem. The resistor network exception was created to allow stations to reduce their antenna efficiency without having to modify the existing tower.
Rule changes in the 1980s did away with the fixed set of power choices, allowing stations to choose an appropriate power level for their antenna system ("dial-a-power"), so there should no longer be any need for the concept of nominal power. However, stations still take advantage of the resistor exception in some cases, simply because they perceive the marketing
advantage of higher power (or at least "round" power) to be worth the cost of the wasted energy
.
See also: effective radiated power
, the regulatory analogue for VHF
and UHF
broadcasting
Measurement
Measurement is the process or the result of determining the ratio of a physical quantity, such as a length, time, temperature etc., to a unit of measurement, such as the metre, second or degree Celsius...
of a mediumwave
Mediumwave
Medium wave is the part of the medium frequency radio band used mainly for AM radio broadcasting. For Europe the MW band ranges from 526.5 kHz to 1606.5 kHz...
radio station
Radio station
Radio broadcasting is a one-way wireless transmission over radio waves intended to reach a wide audience. Stations can be linked in radio networks to broadcast a common radio format, either in broadcast syndication or simulcast or both...
's output used in the United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
. AM broadcast
Broadcasting
Broadcasting is the distribution of audio and video content to a dispersed audience via any audio visual medium. Receiving parties may include the general public or a relatively large subset of thereof...
ers are licensed by the Federal Communications Commission
Federal Communications Commission
The Federal Communications Commission is an independent agency of the United States government, created, Congressional statute , and with the majority of its commissioners appointed by the current President. The FCC works towards six goals in the areas of broadband, competition, the spectrum, the...
to operate at a specific nominal
Real versus nominal value
In economics, nominal value refers to a value expressed in money terms in a given year or series of years. By contrast, real value adjusts nominal value to remove effects of price changes over time...
power
Power (physics)
In physics, power is the rate at which energy is transferred, used, or transformed. For example, the rate at which a light bulb transforms electrical energy into heat and light is measured in watts—the more wattage, the more power, or equivalently the more electrical energy is used per unit...
, which may be (and usually is) different from the transmitter power output
Transmitter power output
In radio transmission, transmitter power output is the actual amount of power of radio frequency energy that a transmitter produces at its output....
.
- For non-directionalOmnidirectional antennaIn radio communication, an omnidirectional antenna is an antenna which radiates radio wave power uniformly in all directions in one plane, with the radiated power decreasing with elevation angle above or below the plane, dropping to zero on the antenna's axis. This radiation pattern is often...
stations, nominal power is normally equal to the RFRadio frequencyRadio frequency is a rate of oscillation in the range of about 3 kHz to 300 GHz, which corresponds to the frequency of radio waves, and the alternating currents which carry radio signals...
power presented to the antenna, as determined from the base current and the antenna's nominal impedanceElectrical impedanceElectrical impedance, or simply impedance, is the measure of the opposition that an electrical circuit presents to the passage of a current when a voltage is applied. In quantitative terms, it is the complex ratio of the voltage to the current in an alternating current circuit...
at the carrierCarrier waveIn telecommunications, a carrier wave or carrier is a waveform that is modulated with an input signal for the purpose of conveying information. This carrier wave is usually a much higher frequency than the input signal...
frequencyFrequencyFrequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...
. - For directionalDirectional antennaA directional antenna or beam antenna is an antenna which radiates greater power in one or more directions allowing for increased performance on transmit and receive and reduced interference from unwanted sources....
stations, nominal power is normally equal to the RF power at the common point (the point at which the transmitter output branches off into separate phasing networks for each towerTowerA tower is a tall structure, usually taller than it is wide, often by a significant margin. Towers are distinguished from masts by their lack of guy-wires....
).
In both cases, nominal power excludes losses in transmission lines between the tower or phasor
Phasor
Phasor is a phase vector representing a sine wave.Phasor may also be:* Phasor , a stereo music, sound and speech synthesizer for the Apple II computer* Phasor measurement unit, a device that measures phasors on an electricity grid...
and the transmitter; however, it includes losses in a resistor network used to decrease the efficiency of the antenna system.
Nominal power is ultimately a historical artifact of the regulatory regime employed by the FCC prior to the 1980s. In the old system, rather than allowing licensees to choose any power level which would meet the efficiency and interference
Interference (communication)
In communications and electronics, especially in telecommunications, interference is anything which alters, modifies, or disrupts a signal as it travels along a channel between a source and a receiver. The term typically refers to the addition of unwanted signals to a useful signal...
standards for their class, stations were restricted to a small set of power levels: 50, 100, 250, 500, 1000, 2500, 5000, 10000, 25000, and 50000 watt
Watt
The watt is a derived unit of power in the International System of Units , named after the Scottish engineer James Watt . The unit, defined as one joule per second, measures the rate of energy conversion.-Definition:...
s. A station whose maximum coverage would otherwise be available at 4500 watts (given a specific directional pattern and antenna system efficiency) had a choice of either living with 2500 watts, or reducing the antenna efficiency to a level which would allow for 5 kW. Newly-constructed stations could fairly easily design an antenna system to meet the requirements, but stations on or moving to a shared tower with higher efficiency had a problem. The resistor network exception was created to allow stations to reduce their antenna efficiency without having to modify the existing tower.
Rule changes in the 1980s did away with the fixed set of power choices, allowing stations to choose an appropriate power level for their antenna system ("dial-a-power"), so there should no longer be any need for the concept of nominal power. However, stations still take advantage of the resistor exception in some cases, simply because they perceive the marketing
Marketing
Marketing is the process used to determine what products or services may be of interest to customers, and the strategy to use in sales, communications and business development. It generates the strategy that underlies sales techniques, business communication, and business developments...
advantage of higher power (or at least "round" power) to be worth the cost of the wasted energy
Energy
In physics, energy is an indirectly observed quantity. It is often understood as the ability a physical system has to do work on other physical systems...
.
See also: effective radiated power
Effective radiated power
In radio telecommunications, effective radiated power or equivalent radiated power is a standardized theoretical measurement of radio frequency energy using the SI unit watts, and is determined by subtracting system losses and adding system gains...
, the regulatory analogue for VHF
Very high frequency
Very high frequency is the radio frequency range from 30 MHz to 300 MHz. Frequencies immediately below VHF are denoted High frequency , and the next higher frequencies are known as Ultra high frequency...
and UHF
Ultra high frequency
Ultra-High Frequency designates the ITU Radio frequency range of electromagnetic waves between 300 MHz and 3 GHz , also known as the decimetre band or decimetre wave as the wavelengths range from one to ten decimetres...
broadcasting